Oil sand is essentially a matrix of bitumen, mineral material and water, although encapsulated air may also be present. The bitumen component of oil sand consists of viscous hydrocarbons which behave much like a solid at normal in situ temperatures and which act as a binder for the other components of the oil sand matrix. The mineral matter component of oil sand typically consists largely of sand, but may also include rock, silt and clay. Sand and rock are considered to be coarse mineral matter, while clay and silt are considered to be fine mineral matter, where fines are defined as mineral matter having a particular size of less than 44 microns. The water component of oil sand consists essentially of a film of connate water surrounding the sand in the oil sand matrix, and may also contain particles of fine mineral matter within it.
A typical deposit of oil sand will contain about 10% to 12% bitumen and about 3% to 6% water, with the remainder of the oil sand being made up of mineral matter. Typically the mineral matter component in oil sand will contain about 14% to 20% fines, measured by weight of total mineral matter contained in the deposit, but the amount of fines may increase to about 30% or more for poorer quality deposits. Oil sand extracted from the Athabasca area near Fort McMurray, Alberta, Canada, averages about 11% bitumen, 5% water and 84% mineral matter, with about 15% to 20% of the mineral matter being made up of fines.
Oil sand deposits are mined for the purpose of extracting bitumen from them, which is then upgraded to synthetic crude oil. Currently, the most widely used process for extracting bitumen from oil sand is the "hot water process", in which both aggressive thermal action and aggressive mechanical action are used to liberate and separate bitumen from the oil sand. The hot water process is a three step process. First, the oil sand is conditioned by mixing it with hot water at about 95.degree. Celsius and steam in a conditioning vessel which vigorously agitates the resulting slurry in order to completely disintegrate the oil sand. Second, once the disintegration is complete, the slurry is separated by allowing the sand and rock to settle out, and the bitumen, having air entrained within it, floats to the top of the slurry and is withdrawn as a bitumen froth. Third, the remainder of the slurry, which is referred to as the middlings, is then treated further or scavenged by froth flotation techniques to recover bitumen that did not float to the top of the slurry during the separation step.
The goal in the hot water process is to recover as much of the bitumen as possible before scavenging the middlings, since the middlings include most of the fines that were contained in the oil sand, but in a dispersed state, which tends to hold them and the remaining bitumen in suspension, thus making the recovery of the bitumen during the scavenging step quite difficult.
To assist in the recovery of bitumen during the separation step, sodium hydroxide (caustic) is typically added to the slurry during the conditioning step in order to maintain the pH of the slurry slightly basic, in the range of 8.0 to 8.5. This has the effect of chemically dispersing the clay that becomes dispersed in the slurry during the conditioning step, which in turn reduces the viscosity of the slurry by reducing the particle size of the clay minerals present in the slurry. With the clay present in the slurry chemically dispersed and the viscosity of the slurry lowered, the bitumen more readily floats to the surface of the slurry and can therefore be more readily recovered during the separation step.
There are several disadvantages to the hot water process. The use of hot water and steam in the process, as well as the vigorous agitation to which the oil sand is subjected during the conditioning step, mean that the energy requirements of the process are very high. In addition, since the main goal of the hot water process is to liberate and separate bitumen from the oil sand by completely destroying the oil sand matrix, most of the fine mineral matter contained in the oil sand becomes mechanically dispersed throughout the slurry during the conditioning step.
The addition of caustic to the slurry to reduce the viscosity of the slurry results in further chemical dispersal of the clay in the fine mineral matter, whereby the size of the individual clay particles may be reduced to as small as 0.2 microns. The combination of the vigorous and complete physical dispersal of the fines contained in the oil sand and the chemical dispersal of the clay in the resulting slurry create a middlings stream that may contain a large amount of very well dispersed fines held in suspension, particularly where the oil sand deposit is of lower quality and therefore has a relatively high fines content. As the fines content of the oil sand feedstock increases, the concentration of fines in the slurry increases, and recovery of bitumen from the slurry becomes more difficult, since the suspended fine particles tend to "trap" bitumen within the slurry.
In addition to the problems regarding the recovery of bitumen from slurries containing a large amount of dispersed fines, the middlings stream that remains following the scavenging step poses a huge disposal problem, since it constitutes a sludge that tends to settle and consolidate very slowly. Current practice for the disposal of the sludge remaining after the scavenging step involves pumping it into huge tailing ponds, where the fines slowly settle and stratify. After several weeks, some of the water forming the sludge will be present at the top of the tailing pond containing only a small amount of suspended fines. This water may be recycled for use in the hot water process, after being reheated to the process temperature. The remaining sludge continues to settle and gradually increases in density until over a period of perhaps 10 years, the solids concentration of the sludge may increase to up to 50%. Complete settlement and consolidation of the sludge may take in the order of hundreds of years. It is thought that the reason for the slow consolidation of the sludge is that the clays that become physically and chemically dispersed during the hot water process partially reflocculate into a fragile gel network, through which fines that are larger than the clay particles gradually settle.
In any event, because of the characteristics of the middlings sludge, the tailing ponds cannot be completely rehabilitated for many, many years, and only a portion of the water that enters the tailing ponds can be recovered and reused in the hot water process, thus creating a requirement that a large amount of makeup water be available for the hot water process to make up for the water that is lost to the tailing ponds.
Some attempts have been made to improve upon the hot water process. Canadian Patent No. 1,085,761 (Rendall) issued on Sep. 16, 1980 discloses a process for extracting bitumen from oil sand that entails showering the oil sand through a bath of hot water while passing the oil sand and hot water through the conditioning vessel countercurrent to each other. To assist in the separation of bitumen from the resulting slurry, the addition of caustic or a polyphosphate as a froth suppressing agent is taught. It is claimed that this invention reduces the amount of process water required from the amount used in the typical hot water process, thus reducing the energy requirements of the process. However, this patent does not address the effects of physical dispersal of fine material by agitation of the oil sand, or chemical dispersal of clay by the addition of caustic to the slurry. It also does not address the high energy requirements necessitated by the vigorous agitation of the oil sand and the use of a high process temperature.
U.S. Pat. No. 4,512,956 (Robinson et al), issued on Apr. 23, 1985, and U.S. Pat. No. 4,533,459 (Dente et al), issued on Aug. 6, 1985, disclose respectively, an apparatus for extracting bitumen from oil sand, and a process utilizing the apparatus. These patents recognize the desirability of minimizing the physical dispersal of fines during the extraction process and offer as a solution the conditioning of oil sand with a large amount of hot process water, while at the same time minimizing the mechanical action during the conditioning step so that the oil sand is not substantially disintegrated. A combination of the high ratio of water to oil sand in the slurry, the gentleness of the conditioning step, and the addition of caustic to regulate the pH of the slurry is stated to improve the recovery of bitumen from the oil sand, as well as reduce the energy requirements as compared with the hot water process, since more process water can be recycled and less energy is expended in gently conditioning the oil sand. However, the process disclosed by these patents still makes use of hot water, and at the high ratios of water to oil sand prescribed by the patents, the thermal energy requirements are very significant. Furthermore, because caustic or another reagent is added in order to adjust upwards the pH of the slurry, the effects of chemical dispersal of the clays in the slurry will make the resulting middlings sludge difficult to dispose of, even if the relative amount of fines in the slurry is reduced as compared with the hot water process.
Canadian Patent Application No. 2,030,934 (Strand), filed on Nov. 27, 1990, and corresponding Patent Cooperation Treaty Patent Application No. PCT/CA91/00415, filed on Nov. 22, 1991, both describe an extraction apparatus and process employing a countercurrent separator vessel in which oil sand is gently rolled from one end to the other by a spiral ribbon and mixer elements while hot water, defined as having a temperature of 50.degree. Celsius circulates in the opposite direction. Two streams are then removed from opposite ends of the separator vessel. One stream contains coarse material and some water, while the other stream contains bitumen and dispersed fines in a slurry. Mechanical action is minimized and liberation and separation of bitumen is accomplished almost entirely by thermal action. It is stated in these applications that an important objective of the invention is to leave most of the clay in the oil sand in its original state so that it may be returned along with separated coarse material, to the site from which the oil sand was mined. It is also stated that due to limited dispersal of clay in the process water, it should not normally be necessary to add caustic to aid in the recovery of bitumen, and a substantial portion of the process water will be available for recycling. As for the amount of process water required, it is stated that the water to oil sand ratio is a function of the heat transfer requirements of the system, and not the requirement to provide adequate dilution of the slurry to facilitate bitumen recovery.
Other attempts have been made to improve upon current practises for disposing of and rehabilitating the solid material and sludge that is generated during the hot water process. U.S. Pat. No. 4,414,117 (Yong et al), issued on Nov. 8, 1983, relates to the discovery that clay sludge will settle more quickly if carbonate and bicarbonate ions are first removed from the sludge. The patent teaches that the removal of carbonate and bicarbonate ions can be accomplished in several ways, such as by the use of an ion exchange resin, the addition of an ion precipitant, or the use of a mineral acid such as hydrochloric acid to convert the carbonate and bicarbonate ions to CO.sub.2. It is stated that best results are obtained when essentially all of the carbonate and bicarbonate ions are removed from the system with the result being that the settlement rate of the sludge is significantly accelerated.
Similarly, it has been observed that the microstructure of fine oil sand tailings is the subject of a reversible process and it has been postulated that the microstructure can be controlled by controlling both the pH and bicarbonate ion concentration in the tailings (Sheeran, D., Sethi, A., and Smith, P., An Integrated Approach to Environmentally Acceptable Disposal of Athabasca Oilsand Fine Tailings, Joint CSCE-ASCE National Conference on Environmental Engineering, Jul. 12-14, 1993, Queen Elizabeth Hotel, Montreal, Quebec).
U.S. Pat. No. 4,225,433 (Liu et al) relates to a process whereby the solid material and the sludge that is generated during the hot water process is combined together, mixed with a flocculating agent and then vacuum filtered to separate water and solid material. The patent indicates that the fines form agglomerates with the coarse particles and that the agglomerates settle at a rate comparable to that of the solid material alone.
Other efforts have focused on the characteristics of the solid tailings and sludge tailings as a whole, and in particular, the feasibility of combining them together to create a waste stream that is easier to handle. (Scott, J. D., and Cymerman, G. J., Prediction of Viable Tailings Disposal Methods, Proceedings of a Symposium: Sedimentation Consolidation Models, ASCE/October, 1984, San Francisco, Calif.). This paper indicates that tailings typically produced by Syncrude Canada Ltd. at Fort McMurray, Alberta tend to be a segregating mix so that the solid material settles out from the tailings quickly, leaving the sludge. Segregation is detrimental due to the problems associated with the disposal of the sludge. To prevent fines segregation, it is stated that it is necessary to lower the water content of the tailings stream, increase the fines content of the tailings, or do both. Based upon this analysis, the authors conclude that promising proposals include the mixing of mature sludge taken from the bottom of tailing ponds with a thickened sand tailing to produce a nonsegregating mix, or the mixing of sand, sludge and overburden stripped from the mine site in order to produce a nonsegregating mix.
It can therefore be seen that the challenge in extracting bitumen from oil sand is to maximize the recovery of bitumen while minimizing the amount of sludge that is generated, and while controlling the physical characteristics of the sludge so that it may be more easily disposed of. Also desirable is to minimize the energy requirements of the process as much as possible so that the process can be carried out in an economical and environmentally acceptable manner.
More specifically, the goal is to eliminate the need for sludge tailing ponds which typically occupy many square kilometres, and replace the sludge currently disposed of in these tailing ponds with nonsegregating tailings produced from both the solid material generated by the extraction process and the sludge generated by the extraction process. In order to minimize the energy requirements of the process, it is necessary both to limit the thermal and mechanical energy input into the process and to limit the amount of thermal energy that is lost during the process to the various product and waste streams.